Optical transmission system

Abstract

An optical transmission system is provided in which a plurality of Raman amplifiers are combined so that the wavelength band of a first Raman amplifier in the form of an upward convex curve including the maximum gain value and the wavelength band of a second Raman amplifier in the form of a downward convex curve including the minimum gain value overlap with each other, and that the wavelength band of the first Raman amplifier in the form of a downward convex curve including the minimum gain value and the wavelength band of the second Raman amplifier in the form of an upward convex curve including the maximum gain value overlap with each other. This arrangement also applies to cases where three or more amplifiers are used. Due to this arrangement, an optical transmission system is realized in which it is possible to attain a reduction in Raman gain flatness without using any means such as an equalizer.

What is claimed is:  
     
       1. An optical transmission system comprising: 
       a plurality of Raman amplifiers in cascade with each other including a plurality of pump light sources of different pump wavelengths;  
       said plurality of Raman amplifiers have their several optical fibers where stimulated Raman scattering is generated;  
       said Raman amplifiers mutually compensate for their wavelength dependence of Raman gain; and  
       the total Raman gain flatness is smaller than the cumulative flatness of the assumption that every amplifier has the same gain profile of one of said Raman amplifiers, wherein said plurality of Raman amplifiers are designed to be of at least two types of pump wavelength sets; one of said Raman amplifiers comprises the same pump wavelength set and the same kind of amplifier fiber as another, but their gain levels are different.  
     
     
       2. An optical transmission system comprising: 
       a plurality of Raman amplifiers in cascade with each other including a plurality of pump light sources of different pump wavelengths;  
       said plurality of Raman amplifiers have their several optical fibers where stimulated Raman scattering is generated;  
       said Raman amplifiers mutually compensate for their wavelength dependence of Raman gain; and  
       the total Raman gain flatness is smaller than the cumulative flatness of the assumption that every amplifier has the same gain profile of one of said Raman amplifiers wherein,  
       at least a first Raman amplifier and a second Raman amplifier are used as said plurality of Raman amplifiers;  
       a wavelength band where the Raman gain wavelength property of said first Raman amplifier exhibits an upward convex curve including a maximum value of Raman gain of G Amax  at a wavelength of λ Amax  and a wavelength band where the Raman gain wavelength property of said second Raman amplifier exhibits a downward convex curve including a minimum value of Raman gain of G Bmin  at a wavelength of λ Bmin  overlap with each other;  
       a wavelength band where the Raman gain wavelength property of said first Raman amplifier exhibits a downward convex curve including a minimum value of Raman gain of G Amin  at a wavelength of λ Amin  and a wavelength band where the Raman gain wavelength property of said second Raman amplifier exhibits an upward convex curve including a maximum value of Raman gain of G Bmax  at a wavelength of λ Bmax  overlap with each other; and  
       the total Raman gain flatness is smaller than the flatness of said Raman amplifier and said second Raman amplifier.  
     
     
       3. An optical transmission system according to  claim 2 , wherein the wavelength λ Amax  and the wavelength λ Bmin  are substantially equal to each other, and the wavelength λ Amin  and the wavelength λ Bmax  are substantially equal to each other. 
     
     
       4. An optical transmission according to  claim 1  wherein at least one of a discrete Raman amplifier and a distributed Raman amplifier is used as said Raman amplifier.